Solution
= Solution
{c}
Label the outer square $000,001,011,010$ in cyclic order and the inner square $100,101,111,110$ in corresponding order. Join corresponding vertices radially. Each edge changes exactly one bit, so this gives every edge of $Q_3$, with no crossings.
\Image[/past-exam-of-the-probability-statistics-and-options-i-course-of-the-university-of-oxford/2009-cube.png]
{title=A labelled planar cube with edge colours determined by the coordinate that changes}